The enamel slip currently employed in the art usually contains an enamel frit, electrolytes, a dispersing agent and water. In addition, the composition of such enamel slip can incorporate quartz sand, feldspar for the purpose of increasing thermal and chemical resistance, as well as to enlarge the temperature range of the coating baking.
As the basic suspending agent clay is generally used which ensures a suspended state of the frit particles and imparts a predetermined consistence to the slip which is necessary for the application thereof onto the surface of metal articles. The clay-containing enamel slip is applied to articles by various methods such as dipping, casting, pulverization and the like. Thereafter, the coated article is subjected to baking, whereupon the coating is fused. Clay incorporated in the slip composition requires a higher baking temperature and causes such coating defects as bubbles, pores, "fish-scaling". Furthermore, clay does not have a stable composition which hinders the manufacture of a slip with stable properties. Due to rather limited sources of clay deposits, clay is not a readily available material. For this reason, in the art there is a persistent need in materials which could substitute for clay as a slip component. The use of bentonite, urea and methanol as a suspension agent is known in the art (cf. U.S. Pat. No. 2,824,458 Cl. 117-70, Int. Cl. C 23 d. 1958). The enamel slip composition disclosed in this U.S. Patent is employed for glass coating. This slip does not have stable properties and can be applied to articles only by spraying, while such coating methods as dipping and casting cannot be employed for application of the slip on metal articles. A slip composition is also known in the art, wherein as the suspending agent use is made of water-soluble high-molecular polymers (cf. French Pat. No. 2,002,314, Int. Cl. C 23 d 5/00, 1969). However, the polymer-containing slip is unstable and may be applied onto articles only by the spraying method.
Suspending agents according to the above-mentioned Patents do not allow a lowered baking temperature.
U.S. Pat. No. 3,278,332 Cl. 117-129, 1966, teaches the use, in a slip composition, of colloidal silica instead of clay; though the use of this material is limited by the high cost thereof; furthermore, silica does not lower the baking temperature.
An enamel slip composition is known in the art which contains fine fibers of asbestos and glass wool (cf. U.S. Pat. No. 2,563,502). However, the production of a slip with stable properties is hindered due to the unstable composition of thin-fiber materials.
A slip composition incorporating, in addition to other ingredients, palygorskite (USSR Inventor's Certificate No. 558880, Cl. CO3 c 7/00, 1977) is also known. This slip has a disadvantage residing in a high temperature of baking of the coating due to a high refractivity of palygorskite.